Embodiments of the invention relate generally to computed tomography (CT) imaging and, more particularly, to a composite material pre-patient x-ray collimator for use as part of a CT imaging system and a method of manufacturing thereof.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis, which ultimately produces an image.
In operation, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. The x-ray source is typically in the form of an x-ray tube that emits x-rays at a focal point, with the x-rays being emitted along diverging linear paths in an x-ray beam. A pre-patient collimator is employed for shaping a cross-section of the x-ray beam and for directing the shaped beam through the patient and toward the detector array. The detector array typically includes a collimator for collimating x-ray beams, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
In CT imaging systems, the pre-patient collimator used for shaping the x-ray beam has historically been constructed by machining a monolithic piece of tungsten. Forming the pre-patient collimator from tungsten was appropriate because of the material's radiation blocking ability and structural properties. However, it is recognized that tungsten is an expensive material and difficult to machine. Additionally, in newer CT imaging systems that implement larger patient coverage, faster rotation speed, and larger bore sizes, pre-patient collimators formed from tungsten become even less desirable. That is, in such newer CT imaging systems, the centripetal acceleration (i.e., G-load) increases dramatically on the pre-patient collimator due to the increasing radius from the center of rotation, faster rotation speed of the components in the gantry, and larger pre-patient collimator size needed to block the beam in large-coverage systems. The weight and forces imposed on the pre-patient collimator are of concern as it affects dynamic balance of the CT imaging system, as well as agility of motion for the collimator.
Lead has also been recognized as a possible material from which to construct a pre-patient collimator, as lead also exhibits ideal radiation blocking capabilities associated with its material density. Unfortunately, similar to the use of tungsten pre-patient collimators, the high density of lead means that a pre-patient collimator constructed of lead is affected by the G-load increase in newer CT imaging systems. Additionally, lead is recognized as being too soft to be useful as a monolithic material and is not compliant under the Restriction of Hazardous Substances Directive (RoHS).
Therefore, it would be desirable to design a pre-patient collimator that combines the blocking power of a high-density material with the structural support of a lower density substrate material, therefore cutting back on weight and cost of the collimator, while preserving robustness, radiation blocking ability, and RoHS compliance.